In U.S. Pat. Nos. 3,824,002 issued to Terry D. Beard on July 16, 1974 and in U.S. Pat. No. 3,976,361 issued to L. M. Fraas et al on Aug. 24, 1976, there are disclosed alternating current liquid crystal light valves which include a photo-sensitive substrate and a liquid crystal electro-optical medium. An image is applied as an input to the photo-sensitive substrate and resulting electrical signals are applied across the liquid crystal layer to modulate projection light which is transmitted through the liquid crystal layer to a dielectric mirror from which it is reflected back out of the liquid crystal layer. In these light valves it is necessary to use alternating current fields because the dielectric mirror used to reflect the projection light is an insulator that blocks a DC potential from the liquid crystal. A metal mirror could be used instead of the dielectric mirror to facilitate switching DC, but then the mirror would have to be a matrix of metal pads (to avoid shorting of all of the image signal in the photoconductor plane), and a matrix mirror creates a number of unwanted and very difficult device problems which in practical devices using liquid crystal layers of a thickness of 2 microns or less, would be almost insuperable.
The present invention overcomes these problems and permits the use of DC driving voltages in a reflection mode light valve by the use of a light-blocking layer and a dielectric mirror which have anisotropic conductivity. Thus, they maintain the high sheet resistivity of an ordinary dielectric layer, while providing high DC conduction perpendicular to the film plane.
In order to maintain independence between the read out function and the input function of light valves or similar devices, one must maintain the device as a reflection mode light valve. Thus, it must have an internal mirror between the electro-optic medium and the substrate. With the present invention one can obtain high DC conductivity through the mirror and very high resistivity along the surface of the mirror. One disadvantage of the matrix approach suggested above is that it is optically visible at the magnifications that are involved when the light valve is used to project an image onto a large screen. More importantly, the edges of the matrix element form discontinuities across which it is extremely difficult to obtain uniform liquid crystal alignment such as is necessary in birefringence type devices. Also, those edges cause enhanced electric fields which reduce device lifetime. Finally, the matrix mirror approach creates a projection light leakage problem through to the photoconductor at the interstices in the array of matrix elements.
It is an object of the present invention to provide a light blocking and/or light reflecting element for use as an interface between a photosensitive substrate and an electro-optical medium in electro-optical devices such as a liquid crystal light valve, which interface overcomes the above-noted problems of previously recognized alternatives.